Summary/Abstract The proposed research seeks to examine differences in children?s aggressive behavior problems (ABP) following a change in family composition (e.g., divorce), what explains these differences, and whether these explanations differ by marital status. ABP in children, such as picking fights, are of increasing concern to practitioners and researchers because when ABP persist past the toddler years, children have more difficulty learning in school, forming long- lasting friendships, and maintaining employment later in life. There is correlational research that suggests that family composition is associated with children?s aggressive behaviors; nonmarital and unstable families (e.g., who have had a change in family composition) tend to have children with more aggressive behavior problems than married, stable families. However, questions remain about the mechanisms behind this association that might account for variability in children?s outcomes and across studies. To address this gap, we propose a study with the Fragile Families and Child Wellbeing data. Using a structural equation model framework, we will test whether changes in family functioning (co-parenting support, father involvement) and economic stress (income, perceptions of economic hardship) explain why changes in family composition are associated with changes in aggressive behavior, and whether there are group differences by marital status. Understanding the consequences of changes in family composition and possible points of intervention will improve program efforts and child wellbeing. One ultimate goal of healthy marriage/relationship and responsible fatherhood programming is to help couples become more stable ? both financially and in terms of their relationships ? to ultimately improve child wellbeing. Thus, studies that seek to understand the healthy relationship and responsible fatherhood associations with economic stability, family functioning, and child wellbeing help address these important public health concerns.